1. Field of the Invention
The present invention relates to a servo pattern writing apparatus/method for writing a servo pattern on a recoding medium using an amplitude method and a servo pattern reading apparatus/method for reading a servo pattern that has been written on a recording medium using an amplitude method and, more particularly, to a technique for compensating the displacement of the position at which a servo pattern is written.
2. Description of the Related Art
When a servo pattern is to be written on a recording medium such as a magnetic disk, a facility or apparatus provided for its purpose is set so as not to be affected by vibration from outside. Further, various measures such as a reduction in wind disturbance inside the apparatus body are taken. Nevertheless, there still exits some influence of the disturbance, causing the head position at the time of writing a servo pattern to be slightly displaced from the ideal position in the vertical position of the track. The servo pattern written in this state does not exhibit the ideal position. Assuming that the ideal track is a one that forms a perfect circle, fine irregularities exist in the shape of the circumference of the circle. The irregularities that have been written on the medium periodically occur at the reading time, so that this phenomenon is referred to as “Repeatable Run-Out (RRO)”.
A magnetic disk drive that uses a magnetic disk as a recording medium uses a magnetic head 32 to store information in one or more disk-like recording media (magnetic disks) 31, as shown in FIG. 11. In order to position the head 32 to a given target position accurately, a servo pattern 33 is written on the recording medium, as shown in FIG. 12. The servo pattern 33 typically has a configuration as shown in FIG. 13. As can be seen from FIG. 13, an amplitude servo pattern is written in a location 34 denoted by “Position” and is used for precise position measurement.
FIG. 14 shows an example of a servo pattern using an amplitude method (amplitude servo pattern). The amplitude servo pattern typically includes four burst patterns of burst A, burst B, burst C, and burst D, as shown in FIG. 14. For the sake of simplicity, only burst patterns A and B are used for the following description of servo pattern writing operation.
The burst A is first written as shown in FIG. 15A and then the burst B is written after the position of a write head is shifted as shown in FIG. 15B. A part of the burst A is erased by being overwritten by the burst B and thereby the edge of the burst A denotes a correct position of the center of a target track together with the newly written burst B.
At the time of reading the servo pattern, the point at which a read head equally straddles the burst A and burst B becomes the center of a target track. If the position of the write head is displaced from the target track center as shown in FIG. 16B at the time when the burst B is written, the position of the boundary between the burst A and burst B is correspondingly shifted to be displaced from the ideal center position. This is repeated in servo information corresponding to one rotation of a track, with the result that RRO is included in a demodulated position signal.
The above-mentioned amplitude servo pattern is written by a dedicated device called an STW (Servo Track Writer). After that, in a state where a recording medium on which the servo pattern has been written is set in, e.g., a magnetic disk drive, the position of the servo pattern is checked, the displacement amount of the servo pattern from the ideal position is calculated, and the calculated amount is written as an RRO compensation value in a post data area in the form of a digital value.
Several tens to several hundreds of servo patters are written for each rotation of a track, and several tens of thousands of tracks are formed on a recording medium. In order to measure the RRO, it is necessary first to determine the positions of a plurality of circumferences whose radii are different from one another and then to measure using the circumferences. Therefore, it requires a huge amount of time to create data for RRO compensation, which become bottleneck for mass production.
As another method, there is known one that changes the length of the burst pattern to be integrated to thereby compensate a displacement in the track direction [refer to, e.g., Patent Document 1 [U.S. Pat. No. 5,907,447] (FIG. 13)].
Further, as still another method, there is known one that uses a phase servo pattern. This method writes the servo pattern while adjusting the phase thereof to thereby compensate the displacement in the track direction (refer to, e.g., Patent Document 2 [Jpn. Pat. Appln. Laid-Open Publication No. 10-172254] (FIG. 17 and subsequent figures)].
As described above, the method that uses an amplitude servo pattern has a disadvantage that it requires a huge amount of time to create data for RRO compensation.
Further, in the method disclosed in Patent Document 1, it is necessary to ensure the area for the burst pattern up to the size required for compensating the burst pattern at the maximum level. The ensured area typically becomes an unused area, thereby significantly deteriorating area efficiency. Further, the method disclosed in Patent Document 2 has disadvantages that a servo writing system to be used is limited to the phase servo system and effect produced by the compensation is small.